Scope: Aquariums & Tanks = the water-holding vessel itself (glass/acrylic tanks, nano/pico bowls, drilled reef-ready tanks, kits bundling tank+stand+basic equipment), not loose filters/heaters/lighting (aquatic-equipment), substrate/decor (aquatic-decor), or land-side terrariums (terrariums) — those are sibling leaves.

Seed:
1. Equivalent of steel_hardness: tank volume (litres) is the one-side-of-a-trade-off numeric — more volume buffers temperature/chemistry swings but multiplies weight, footprint, and required stand strength; nano/pico volumes swing fast and punish mistakes, like a hard edge rewarding skill and punishing impact.
2. A reef-focused aquarist would notice drilled/sump-ready plumbing and material (glass vs acrylic) before the gallon number on the box, since those decide whether the tank can actually run the filtration a goal needs.

Sources:
[wikipedia_aquarium] https://en.wikipedia.org/wiki/Aquarium — glass vs acrylic trade-offs, construction, shapes, water weight, nano size, stocking-rule disagreement — fetched text, curl+strip-tags, verified in scratch/wikipedia_aquarium.txt
[wiki_reef_aquarium] https://en.wikipedia.org/wiki/Reef_aquarium — drilled/reef-ready, sump, overflow styles, nano/pico reef sizes, kits — fetched text, verified in scratch/wiki_reef.txt
[wikipedia_salinity] https://en.wikipedia.org/wiki/Salinity — brackish water salinity range 0.5–30 ppt, corroborates aquarium page's PPM figures — fetched text, verified in scratch/en_wikipedia_org_wiki_Salinity.txt
[engineeringtoolbox_water_density] https://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html — water ≈8.34 lb/US gal, corroborates aquarium page's 8.345 lb/gal — fetched text, verified in scratch/engtoolbox_water_density.txt

Note: WebSearch was exhausted this session; direct fetches of retail/brand sites (Petco, Fishlore, Spruce Pets, Marineland, Aqueon, Clear-For-Life, LiveAquaria) returned 403/404 or JS-only shells, and Internet Archive had no snapshots for the two tried. Register is built on two independent primer-grade sources (Wikipedia aquarium + reef aquarium pages) plus two corroborating reference sources for the numeric claims, rather than brand spec sheets.

Branches:
tank_material=glass -> glass_construction (glass's edge/frame finish only applies to glass panes) — wikipedia_aquarium: silicone sealant eliminated the need for a structural frame.
tank_material=acrylic -> acrylic_construction, acrylic_uv_care (seam method and UV stability are acrylic-specific concerns) — wikipedia_aquarium: acrylic-soluble cement fusing, UV stabilization history.
tank_use=reef_saltwater -> drilled_reef_ready (salt setups are the main market for drilled/sump-fed and AIO tanks) — wiki_reef_aquarium: reef-ready tanks drilled for sump/refugium; waterbox_catalog: AIO integrates filtration/overflow into the tank. brackish no longer routes to drilled_reef_ready: wiki_reef_aquarium's "brackish" mentions are bare see-also links, never a discussion of drilling/sump need.
tank_use=planted_aquascape -> makes glass_construction/glass_clarity (rimless, ultra-clear) salient — wikipedia_aquarium: Amano-era aquascaping aesthetic (general link to display-quality glass); waterbox_catalog: CLEAR/CLEAR MINI lines explicitly market "rimless, ultra-clear glass" for aquascaping-style display tanks (the word "rimless" itself is not in wikipedia_aquarium — corrected 2026-10-26, see Corrections).

Noise (spec-sheet-style rows considered, left out):
- Exact glass/acrylic thickness-by-height chart: common on brand spec pages, but no two independently-sourced numeric chart found this run (brand pages blocked); left out rather than inventing a range.
- Turtle/paludarium-specific floor-space-vs-height dimensions: not found in the sources fetched; folded into tank_use's specialist_or_quarantine value only qualitatively.
- Silicone seal color (black vs clear): cosmetic, not a buying differentiator experts track; left out.
- Peninsula/multi-view tank orientation: term not found in fetched reef-aquarium text this run; left out rather than guessing.
- Background-integrated tanks: too thinly sourced this run to state confidently; left out.

Corrections (2026-10-26):
- scale_class/vocab "Nano tank": replaced the invented 40-48 L cutoff (not in either source) with wikipedia_aquarium's actual 75 L figure; noted reef-specific "nano reef" listings run higher, up to ~140 L (wiki_reef_aquarium); fixed the reference supports line that had misattributed the 150 L/40 gal "mini aquarium" figure to "nano."
- Branch tank_use=brackish -> drilled_reef_ready removed (wiki_reef_aquarium's "brackish" mentions are unrelated see-also links); split rationale and tank_use.why_experts_care reworded so brackish implies corrosion-safe parts, not drilling.
- Added facet glass_clarity (standard float vs low-iron ultra-clear), sourced to waterbox_catalog + fkw_acrylic_vs_glass; wired into the tank_material=glass and tank_use=planted_aquascape branches.
- Added facet tank_dimensions (external L x W x H), sourced to waterbox_catalog.
- Expanded drilled_reef_ready's values to drilled_external_sump / integrated_aio_filtration / standard_undrilled, sourced to waterbox_catalog's AIO line; reclassified source from listing to researched and narrowed listing_field_hint to the reef/marine subset of listings.
- Added vocabulary entries for "Quarantine tank" and "Aquascape" (used in tank_use's values/cold_start_question without definition).
- Reworded capacity_volume's range.typical: dropped the unsourced "40-400 L" band, kept only the sourced 1,000 L ceiling (wikipedia_aquarium) and the corrected 75 L nano cutoff.
- scale_class's pico <=10 L cutoff is marked single-sourced (wiki_reef_aquarium only) in its listing_field_hint per house rule, rather than inventing a second source.
- tank_use's brackish salinity figure (0.5-30 ppt) is no longer single-sourced: added wikipedia_aquarium's own "500 to 30,000 PPM" statement as a second supports line on that reference, corroborating wikipedia_salinity.
- Rejected: did not add a separate euro-bracing value to glass_construction or fold pane-repairability into tank_material (check.md's two lower-priority "Minor" items, not on the Required fixes list) — out of scope for this pass.